MAST10007 Linear Algebra Semester 2 Assessment, 2019
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Semester 2 Assessment, 2019
MAST10007 Linear Algebra
Question 1 (9 marks)
Consider the system of equations
x + 2y + 2z = 2
2x + 5y + 3z = 5
x + 3y + k2 z = k + 2
where x, y, z e R and k e R.
(a) Determine the values of k, if any, for which the system has
(i) a unique solution, (ii) no solutions, (iii) infinitely many solutions. (b) Find all solutions to the system when k = 1.
Question 2 (11 marks)
(a) Consider the matrices A = ┌3(1) |
5(2)┐ , B = ┌ 1(2) |
0 _2 |
_41┐ , C = ┌0(1) |
2 5 |
3_1┐ . |
Calculate the following, if they exist:
(i) AB, (ii) BCT .
(b) Prove that if A and B are matrices such that AB and BA are both defined, then AB and BA are both square matrices.
(c) Use cofactor expansion to find the determinant of the matrix
┌┐
' '
where a, b, c, d are complex numbers.
When is A invertible? Explain your answer.
Question 3 (5 marks)
Let
A = ┌1(0) 1
'
1(1)┐
0' .
(a) Verify that A2 _ A = 2I, where I is the 3 × 3 identity matrix.
(b) Deduce from part (a) that A is invertible, and that A一1 = (A _ I).
Question 4 (12 marks)
Consider the points P (1, 1, 0), Q(0, 1, 2) and R(1, 0, 1) in R3 .
(a) Find the distance between P and Q.
(b) Find the cosine of the angle between the vectors PQ and PR.
(c) Find the area of the triangle with vertices P , Q and R.
(d) Find a Cartesian equation for the plane that passes through P , Q and R.
Question 5 (10 marks)
In each part of this question, determine whether W is a subspace of the real vector space V . For each part, give a complete proof using the subspace theorem, or a specific counterexample to show that some subspace property fails.
(a) V = R4 , W = {(a, b, c, d) e R4 | a + b + c + d = 1}.
(b) V = p3 , W = {p(x) e p3 | p(x) = p(_x) for all x e R}.
(c) V = M2,2 , W = {A e M2,2 | A2 = A}.
Question 6 (9 marks)
The following two matrices are related by a sequence of elementary row operations:
┌ 2(1) A = '(')1 '2 |
2 1 0 _1 |
_1 4 3 8 |
3(0)┐' 2 ' , |
┌0(1) B = '(')0 '0 |
0 1 0 0 |
3 _2 0 0 |
2_1┐ 0 ' |
Let W be the subspace of R4 spanned by the set of vectors
S = {(1, 2, 1, 2), (2, 1, 0, _1), (_1, 4, 3, 8), (0, 3, 2, 5)}. (a) Find a subset of S that is a basis for W . Hence, find the dimension of W . (b) Write the other vectors in S as a linear combination of your basis vectors.
(c) Consider the set of vectors
T = {(_1, 4, 3, 8), (0, 3, 2, 5)}.
Is T linearly independent? Is T a basis for W? Explain your answers.
Question 7 (11 marks)
Let T : R3 - R3 be the linear transformation given by
T (x, y, z) = (x + y + 2z, y + 2z, z).
Consider the bases of R3 given by
s = {(1, 0, 0), (0, 1, 0), (0, 0, 1)}
and
B = {(1, 0, 0), (1, _1, 0), (2, _1, _1)} .
(a) Find the matrix [T]S of T with respect to the standard basis s .
(b) Is T (i) injective, (ii) surjective, (iii) invertible? Explain your answers. (c) Find the transition matrix PS ,夕 .
(d) Find the transition matrix P夕 ,S .
(e) Find the matrix [T]夕 of T with respect to the basis B .
Question 8 (13 marks)
Consider the function T : M2,2 - M2,2 given by
T (X) = ┌ 1(2) 1(2)┐ X.
(a) Show that T is a linear transformation.
(b) Find the matrix [T]S of T with respect to the standard basis
s = {┌0(1) 0(0)┐ , ┌0(0) 0(1)┐ , ┌1(0) 0(0)┐ , ┌0(0) 1(0)┐}
(c) Find a basis for the kernel of T.
(d) Find a basis for the image of T.
(e) Verify the rank-nullity theorem for the linear transformation T.
Question 9 (6 marks)
For each of the following matrices, determine whether it is diagonalisable and give a short justification:
A = ┌0(i) |
1 i(+) i┐ , |
B = ┌ _02 ' 0 |
5 3 0 |
1_6┐ _4' , |
C = ┌ _52 ' 1 |
5 _2 _6 |
1_6┐ _2' . |
(Hint: Very little calculation should be needed to answer this question.)
Question 10 (13 marks)
In a certain town, the weather each day is either rainy or fine.
· If the weather is rainy one day, then it is rainy the next day 60% of the time. · If the weather is fine one day, then it is fine the next day 80% of the time.
Let rn be the probability that the weather is rainy after n days, and fn be the probability that the weather is fine after n days.
(a) Explain briefly why
┐ = A ┌ f(r)n(n)┐ ,
where
A = ┐ .
(b) Find the eigenvalues and corresponding eigenvectors for A.
(c) Find an invertible matrix P and a diagonal matrix D such that A = PDP一1 .
(d) Assuming that today is fine we have r0 = 0 and f0 = 1. Find formulas for rn and fn for n > 1.
(e) What are the long term probabilities of rainy days rn and fine days fn, as n - o?
Question 11 (10 marks)
Consider R3 with the standard inner product given by the dot product
(u, v) = u . v = u1v1 + u2v2 + u3v3 .
Let W c R3 be the subspace spanned by
{(0, 1, 1), (1, 0, 1)}.
(a) Find an orthonormal basis for W .
(b) For v = (1, 1, 0) e R3, find
(i) the orthogonal projection of v onto W ,
(ii) the distance from v to W .
Question 12 (7 marks)
(a) Find the least squares line of best fit y = a + bx for the data points
{(_1, 2), (0, 1), (1, 2), (2, 3)}
(b) Draw a clear graph showing the data points and your line of best fit.
Question 13 (4 marks)
Let A be an n × n real matrix. Fix a real number λ and consider the set
W = ,w e Rn | (A _ λI)2 w = 0、.
Show that W {0} if and only if λ is an eigenvalue of A.
2022-10-15